Alloying driven transition between ferro- and antiferromagnetism in UTGe compounds: the UCo1-xIrxGe case

The evolution of magnetic properties of isostructural and isoelectronic solid solutions of the superconducting itinerant 5f-electron ferromagnet UCoGe with antiferromagnet UIrGe was studied by magnetization and specific heat measurements of a series of UCo1-xIrxGe compounds at various temperatures and magnetic fields. The ferromagnetism in UCoGe was found to have vanished already for the lowest studied Ir substitution for Co (x=0.005) whereas superconductivity appears to persist up to x=0.01. The antiferromagnetism of UIrGe is gradually suppressed with decreasing Ir concentration to disappear by x=0.75. The section of the T-x phase diagram in the range of x 0.005-0.75 is dominated by a paramagnetic phase. With increasing Ir concentration for x above 0.4, antiferromagnetic correlations gradually appear in a paramagnetic state, which may eventually lead to frozen incoherent spin configurations at low temperatures. These results are clearly in stark contrast to the behavior of the related URh1-xIrxGe system, which exhibits a discontinuous transformation between the ferromagnetic and antiferromagnetic phases of parent compounds at a critical Rh-Ir concentration. The striking difference is due to the different degrees of the localization of U 5f-electron states in the weak itinerant ferromagnet UCoGe and the local moment ferromagnet URhGe.